Hi,
I "googled" "yamaha FZS600 Fazer gearbox problems or failures", and the first site of "about 10" offers "Lost your front sprocket and washer???-wwwezboard.com". Here's the link -
http://p196.ezboard.com/ffazerownersfrm3.showMessageRange?topicID=2377.topic&start=41&stop=60 Intrigued, I clicked on this and was swept onto the fairly active forum of the Fazer Owners Club (unofficial) where I found, among other things, links to download both parts and service manuals. I tried two links from one of the last posts on that thread, one for a parts manual, the other a service manual. After a little perseverance I finally induced this little device to pay heed to my request of a download of these items and soon there appeared two files, one each for parts and service, opening up in turn in the same window.
These "manuals" provide ample information for the stripdown and rebuild of this motor. Sadly the service manual shows clearly that the top end WILL have to be removed to split the 'cases in order to remove upper crankcase bolts 1 and 2.
Although a bit ambitious for the beginner, I still think that a job like this is within the capabilities of the private owner provided he takes the greatest of care and looks for solutions for the problems he will face. Shortcuts can be taken, special tools are often easily made and cable ties can be useful. Repair it at minimum cost, only replacing parts and gaskets which show damage and taking care to strip and change as little as possible (given that it was running fine beforehand and so should need little doing).
For example, don't remove the pistons from the rods if it is possible to remove and replace the offending bolts with them in situ, bearing in mind the pistons and rings should be treated with care. Check that all rings are free in their grooves by sliding them in and out rather than sliding them round in their grooves, so that the pistons can be refitted with the ring gaps in as close as possible the same position as when they came out.
Any fault found would need to be rectified although a complete rebuild should not be necessary unless it's nacked anyway. You never did tell us the year and mileage of this horse, but the older, less valuable it is, the closer it is to the nacker's yard, and a f**ked gearbox does it. Fix it. You haven't much to lose. Do your research, get your tools organised and get started before it gets worse. But be careful you don't damage it trying to fix it.
All the best, let us know what happens.
A,
I assume the Victor's yours. Well, if you can keep that thing going, you should be able to fix the fazer